5 speed rpms at 70 with 373 and 355s

3.55 is the stock one on those cars manual!

This is a very simple calculation.

Start by determining tire rolling diameter. Example

235 50 18 tire. 23.5CM / 2.54 = 9.25 X .5 = 4.125 X 2 = 9.25 + 18 = 27.25 Diameter

27.25 X Pi = 85.6" / 12 = 7.13

5280 / 7.13 = 740 tire revolutions per mile

740 X 3.55 = 2627 engine revolutions per mile
766 X 3.31 = 2449 engine revolutions per mile

If you travel one mile in one minute that is 60 miles in one hour. Thus this is the number of engine revolutions per minute at 60 MPH.


These calcs are made with top gear being 1 to 1. You will have to factor in the overdrive ratio of fifth gear to get it correct. I THINK that the top gear is .67, thus

.67 X 2627 = 1760
.67 X 2449 = 1640


To get 70 MPH RPM's, simply multiply by the ratio of 60 to 70

70 / 60 = 1.166 Thus 1760 X 1.166 = 2052


NOW, all that described, why are you asking this question?

aleborjas said:

3.55 is the stock one on those cars manual!

Click to expand...

3.55 is the stock gear on the GT manual for 05 & 06 model years. It's a $100.00 option on the 2007.

$87 option (dealer invoice).

MBDiagMan said:

9.25 X .5 = 4.125 X 2 = 9.25

Click to expand...

Not that I'm questioning your math skills but isn't this a little redundant?

here is a handy tire/gear calculator:
http://www.dakota-truck.net/TIRECALC/tirecalc.html

at 70mph